1. (Field of the Invention)
The present invention relates to a method of manufacturing a resin-sealed type semiconductor device and, more particularly, to a method of the resin sealing by melting and injecting a resin tablet in a mold.
2. (Description of Related Art)
The sealing a semiconductor device with a resin is accomplished by using a resin molding machine. This resin molding machine is set with top and bottom forces to form a cavity portion, which is filled with a molten resin by injection. This sealing resin is prepared by forming thermoset resin powder into a cylindrical tablet shape. The bottom force has a pot portion to be charged with the sealing resin; a plunger for pressing and injecting the charged resin onto a cull portion of the top force; and runner portions for guiding the molten resin into the cavity portion. Moreover, the top and bottom forces are individually equipped with heaters for heating and melting the sealing resin. By using the resin molding machine thus constructed, a lead frame on which a semiconductor element is mounted and to which wire bonding is performed is clamped by the top and bottom forces and by injection-molding of the sealing resin the resin-sealed type semiconductor device is fabricated.
This fabrication method will be described in more detail in the following. In the resin injecting portion of the resin molding machine, the plunger is moved downward to an original position, and the space defined by the top face of the plunger and the side wall of the pot portion is charged with the resin tablet. Then, the top and bottom forces are hydraulically clamped. Next, the plunger is moved upward to press the resin tablet onto the cull portion. The injected resin flows through the runner portions into the cavity portion so that the cavity portion is wholly charged with the sealing resin. After lapse of a predetermined time period, the sealing resin is hardened, and the mold is opened to part the sealing resin of the molded piece, the cull portion and the runner portions, thus ending a series of resin molding steps. If necessary, a cleaning step of the mold surfaces may be added. In a conventional method, the process is transferred to the pressure molding step immediately, continuously after the resin molding machine has been charged with the resin tablet.
The fabrication method of the prior art involves following defects. First of all, bonding wires for connecting the bonding pads of a semiconductor element mounted on the island of the lead frame to the inner leads are usually stretched straight when viewed from above but may be greatly curved by the fluid resistance of the sealing resin to cause short-circuit failures. Moreover, the corner portions near the gates of the cavity surfaces of the bottom force always contacts with the new sealing resin flowing through the runners so that they are liable to be blotted with low-molecular weight components of the sealing resin, wax and resin components of unreacted hardening agent or the like. This appears as mold blots and causes uneven surfaces of molded pieces, semiconductor devices, resulting in appearance defect of the products and marking defect.
Blotted mold causes another following problem at the mold parting time. In the parting operation, either of the top and bottom forces is moved relatively apart from each other to open the mold. When the bottom force is moved downward, for example, the ejector pins of the force are simultaneously projected out to part the hardened resin from the top force. Next, when the bottom force drops to its bottom dead center, i.e., the knock-out position, the ejector pins of the bottom force are projected out to separate the sealing resin from the bottom force. In existing fabrication methods, however, the ejector pins are not uniformly raised even when the molded piece is pushed by the ejector pins. As a result, a uniform parting operation cannot be effected to leave the resin in the bottom force or break the resin at the runner portion.